Used to express a range of emotions including surprise, anger, disappointment, or joy, or when reacting to something that has or will soon occur. Used in reference to a 'man' while not necessarily being gender specific to a man.

ORIGIN natural exclamation: first recorded in Australian English in the early 20th cent. Combination of the 'oh' exclamation of mid 16th cent. and Old English 'man(n)' of Germanic origin; related to Dutch man, German Mann, and Sanskrit manu ‘mankind.’